This invention relates generally to semiconductor structures and more particularly to semiconductor structures having reversed-biased junctions to provide isolation between active devices and which are adapted to operate in a radiation environment.
As is known in the art, one technique used to form an integrated circuit is to provide a semiconductor substrate with an epitaxial layer grown thereon, the substrate and the epitaxial having opposite-type conductivities. The substrate is moderately doped to provide a suitable breakdown voltage for active devices formed in the epitaxial layer. Active devices formed in the epitaxial layer are typically electrically isolated from one another by isolation regions extending vertically through the epitaxial layer to the substrate. Such isolation regions may comprise semiconductor material having the same conductivity type as that of the substrate. Alternately, the isolation regions may further comprise an electrical insulator disposed on semiconductor material of the same conductivity type as the substrate. Electrical isolation is achieved by applying a reverse-bias between the substrate and the epitaxial layer.
With such type of isolation, however, when the structure is exposed to radiation, such as x-ray or gamma ray radiation, electron-hole pairs (i.e. majority and minority carriers) having relatively long lifetimes and diffusion lengths are generated in the moderately-doped substrate material. Those relatively-long-diffusion-length carriers which reach the reverse-biased junction between the substrate and the epitaxial layer produce an electrical current which, in addition to the reverse-bias leakage current produced at such reverse-biased junction, reduces the electrical isolation between active devices formed on different regions of the epitaxial layer. Such increased current at such reverse-biased junction also may produce "logic upset", that is, slight conduction of an "OFF" active device in an amount sufficient to prevent an "ON" active device coupled thereto from further conducting, thus turning such device "OFF".
One technique used to reduce the effect of radiation is to use a dielectric material as the substrate. One such material is sapphire, a relatively expensive material, on which it may be difficult to form an epitaxial layer. Another is polycrystaline-silicon, with a silicon epitaxial layer. Generally, however, there are manufacturing difficulties with using an insulating substrate.